Abstract: 2741 aftershocks of the May 12,2008 M_S8.0 Wenchuan earthquake from May 12 to June 26 were relocated using the double difference algorithm to investigate tentatively the seismotectonic deformation and rupture process of the earthquake.Most relocated aftershocks occurred in the upper crust at depths of 0～20km,and fewer in the lower crust from depth of 25 to 40km,which may correspond to faulting in the lower crust.An aseismic layer appears beneath the Longmenshan thrust nappe zone between 20 and 25km depth after relocation,which may be correlated with the detachment for thrusting of the nappe zone.Spatial distribution of the relocated aftershocks and focal mechanism solutions show that the earthquake ruptured from south to north unilaterally with segmentation characteristics.The seismic belt formed by the aftershocks may be divided into two segments by the Gaochuan stepover in Anxian.The width of the seismic belt becomes narrower from south to north in map view,implying that the dip of the seismic fault may become steeper northward.The focal mechanism solutions suggest that rupturing in the south segment where the initial rupture point locates may be dominated by thrust slip with less component of strike slip,while it may have both thrust slip and dextral strike slip in the north segment.From Beichuan to the northeast end of the rupture zone,the seismic belt cannot be correlated with any fault in the north segment of the Longmenshan thrust nappe zone,even it cuts athwart the Qingchuan Fault.We can conclude that no controlling fault in the north segment of the Longmenshan thrust nappe zone was involved in earthquake generating and rupturing.The hypocenters in this part delineate a near vertical rupturing plane perpendicular to the strike of the seismic belt,and the focal mechanism solutions show great dextral strike slip component.

Key words: M_S8.0 Wenchuan earthquake, relocation of aftershocks, double difference algorithm, seismotectonics

摘要： 利用双差地震定位法对5月12日汶川M_S8.0地震至6月26日四川地震台网整理形成观测报告的2741个余震进行了重新定位。在此基础上,初步探讨了汶川地震的地震构造及其余震的破裂扩展。重新定位后震源深度主要分布在0～20km间的上地壳,25～40km的下地壳也有少量地震发生,与下地壳存在脆性变形的断裂活动相对应,在20～25km深度范围内的上下地壳之间存在一个明显的缺震层,推测其可能构成推覆构造的滑脱面。从震源分布与震源机制解在空间的变化上,地震破裂由南向北单侧破裂且存在明显的分段活动性,推测可能存在逆冲推覆与右旋走滑破裂相互转换的过程:逆冲推覆滑动主要发生在高川以南的段落上,震源机制解表现为以逆冲为主;地震破裂向北并未沿龙门山推覆构造带北段扩展,而是斜切青川断裂,震源分布刻画的结构面陡直,震源机制解表现为以右旋走滑错动为主。

关键词: 汶川M_S8.0地震, 余震重新定位, 双差地震定位法, 地震构造